
452 -- DYNAMICAL GEOLOGY. [Boo TIL. 
detritus. Save where protected and concealed under the water of 
lakes, they are everywhere exposed to a renewal of the denudation 
to which they owe their origin. Only where the sediment is strewn — 
over the sea-floor beneath the limit of breaker-action is it permitted 
to accumulate undisturbed. In these quiet depths are now growing 
the shales, sandstones, and limestones, which by future terrestrial 
revolutions will be raised into land, as those of older times have 
been. Between the modern deposits, and those of former sea bottoms 
which have been upheaved, there is the closest parallel. Deposi- 
tion will obviously continue as long as denudation lasts. The 
secular movements of the crust seem to have been always sufficiently — 
frequent and extensive to prevent cessation of these operations. And 
so we may anticipate that it will be for many geological ages yet to 
come. Elevation of land will repair what has been lost by superficial 
waste, and subsidence of sea-level will provide space for continued 
growth of sedimentary deposits. | 
Section III.—Life. 
Among the agents by which geological changes are now, and in 
past time have been effected upon the earth’s surface living organisms 
take by no means an unimportant place. They serve as a vehicle for 
continual transferences from the atmosphere into the mineral world, 
and from the mineral world back into the atmosphere. Thus they 
decompose atmospheric carbon dioxide, and in this process have 
gradually removed from the atmosphere the vast volumes of this gas 
now locked up within the earth’s crust in beds of solid coal. By 
their decomposition organic acids are produced which partly enter 
into mineral combinations, and partly return to the atmosphere as 
carbon dioxide. Plants abstract from the soil silica, alkalies, calcium 
phosphate and other mineral substances which enter largely into 
the composition of the hard parts of animals. On the death and 
decomposition of animals these substances are once more relegated — 
to the inorganic world, thence to enter upon a new circulation 
through the tissues of living organisms. 
From a geological point of view the operations of organic life 
may be considered under three aspects—destructive, conservative, 
and reproductive. | 
§1. Destructive Action. 
Plants in several ways promote the disintegration of rocks, 
1. By keeping the surfaces of rocks moist, they provide means 
for the continuous solvent action of water. This influence is par- 
ticularly observable among liverworts, mosses and similar moisture- 
loving plants. 
2. By their decay they supply an important series of organic acids 
which exert a powerful influence upon soils, minerals and rocks. The 
